Search for gamma-ray emission from eight dwarf spheroidal galaxy candidates discovered in year two of Dark Energy Survey with Fermi-LAT data

Very recently the Dark Energy Survey (DES) Collaboration has released their second group of dwarf spheroidal (dSph) galaxy candidates. With the publicly available Pass 8 data of Fermi-LAT we search for $\gamma$-ray emissions from the directions of these eight newly discovered dSph galaxy candidates. No statistically significant $\gamma$-ray signal has been found in the combined analysis of these sources. With the empirically estimated J-factors of these sources, the constraint on the annihilation channel of $\chi \chi \to {\tau }^{+}{\tau }^{-}$ is comparable to the bound set by the joint analysis of fifteen previously known dSphs with kinematically constrained J-factors for the dark matter mass $m_{\chi }>250\mathrm{GeV}$. In the direction of Tucana III (DES J2356-5935), one of the nearest dSph galaxy candidates that is $\sim 25\mathrm{kpc}$ away, there is a weak $\gamma$-ray signal and its peak test statistic (TS) value for the dark matter annihilation channel $\chi \chi \to {\tau }^{+}{\tau }^{-1}$ is $\approx 6.7$ at $m_{\chi }\sim 15\mathrm{GeV}$. The significance of the possible signal likely increases with time. More data is highly needed to pin down the physical origin of such a GeV excess.

Figure 1

Bin-by-bin integrated energy-flux upper limits at 95% confidence level for each dwarf spheroidal satellite candidates that reported in [52].

Figure 2

The bounds on the $⟨\sigma v⟩$ at 95% confidence level for dark matter annihilation to $b\overline{b}$ (left) and ${\tau }^{+}{\tau }^{-}$ (right) set by the individual DES Y2 candidate dSph as well as by the combined sample, separately. The best limits inferred from a joint analysis of fifteen previously known dSphs with kinematically constrained J-factors [35] is plotted for comparison (see the thick blue line) and the thermal relic cross section [55] is also shown for reference (see the dashed light blue curve).

Figure 3

The TS value of the $\gamma$-ray signal in the direction of Tucana III for different dark matter particle masses and annihilation channels in two different time intervals: The left panel is for 7 year Fermi-LAT data (2008 August 4 to 2014 August 4) and the right panel is for 3.5 year Fermi-LAT data (2008 August 4 to 2012 February 4).

Figure 4

The TS value of the possible dark matter annihilation signal in the combined $\gamma$-ray data in the directions of seven “nearby” dSph galaxies (candidates), including Segue I, Segue II, Ursa Major II, Reticulum II, Tucana III, Cetus II and Willman I. The dark matter annihilation channels are labeled in the plot.